1. Field of the Invention
The present invention relates to an oxygen concentration detector used for air fuel ratio control or the like in an automotive engine.
2. Description of Related Art
As shown in FIG. 14, conventionally, an oxygen concentration detector 9 has a cylindrical housing 10 and a detecting element 12 inserted into and disposed in the cylindrical housing 10. Powdery seal materials 13 made of ceramic powder are filled under pressure between a top portion of the detecting element 12 and the housing 10. The detecting element 12 has an external electrode 121 at an external surface thereof.
The detecting element 12 includes a generally test-tube-shaped solid electrolyte, the external electrode 121 installed at the external surface of the solid electrolyte, an air chamber made inside the solid electrolyte and an internal electrode provided at an interior surface of the solid electrolyte. The external electrode 121 is connected to a lead wire from a connecting piece 181 through a terminal 18 to be conducted to ground. The connecting piece 181 is installed at a lead portion 122 extending toward an upper portion of the detecting element 12 from the external electrode 121.
The internal electrode is connected to the lead wire from a connecting piece 191 through a terminal 19 in such a manner that the internal electrode is connected to an external circuit. The connecting piece 191 is installed at a lead portion 124 extending toward an upper portion of the detecting element 12 from the internal electrode.
Since the oxygen concentration detector 9 has a small size of a few centimeters, a lead wire having a small diameter is used. This kind of lead wire is easily broken, and therefore, an operational malfunction of the oxygen concentration detector 9 occurs. Also, since both the connecting pieces 181 and 191 are disposed close to each other, shorting may occur between the connecting pieces 181 and 191.
In order to prevent the pieces from shorting, a head portion of the detecting element 12 has a stepped structure. However, processing the detecting element 12 takes time.
Conventionally, an oxygen concentration detector in which the detecting element 12 is assembled in the housing 10 in such a manner that the external electrode 121 is in contact with the housing 10 and the oxygen concentration detector 9 is grounded at the housing 10 is disclosed. By providing metallic packing or the like between the external electrode 121 and the housing 10 and using elasticity of the packing or a similar property, electric conduction between the housing 10, the external electrode 121 and the metallic packing can be more reliably ensured.
However, problems in the above-described oxygen concentration detector are found. That is, in the oxygen concentration detector, a front surface of the metallic packing is oxidized because components in the measured gas have high temperatures, and therefore, a conducting malfunction may occur between the external electrode 121 and the housing 10.
Further, when the oxygen concentration detector is installed at a position where oxygen is to be detected, although the oxygen concentration detector is fixed by a screw portion 104 provided at the housing 10, the housing 10 is deformed and a contacting malfunction may occur.
In order to protect the external electrode 121 from corrosive material contained in the measured gas, a protecting layer is usually provided at the external surface of the external electrode 121. Since the protecting layer is a porous film made of ceramic particles or the like, when the protecting layer is extended to a contacting portion with the metallic packing even though the detecting element 12 makes contact with the housing 10 through the metallic packing, electric conduction is not obtained between the detecting element 12 and the housing 10. Without providing the protecting layer, the external electrode 121 may be damaged by the corrosive material.